The present application relates generally to uninterruptible power supplies (UPSs), and more specifically, to a controller for an inverter circuit and a method of use thereof.
Modern power electronic systems, such as uninterruptible power supplies (UPSs), use switched converters coupled with LC filters to remove high-frequency components. The switched converters have an output impedance that represents the ability of the switched converter to maintain a target output voltage under varying load conditions. This is sometimes referred to as output dynamic stiffness. For example, in certain fields, the load varies non-linearly, as in a typical computer load. In other fields, such as medical imaging, the load pulses, in which case a higher output dynamic stiffness is preferred. For a given load, the output dynamic stiffness of an inverter can be characterized by a voltage total harmonic distortion (THD), which is expressed as a percentage. An ideal inverter has zero output impedance and a THD of 0%. The voltage THD is not an absolute measure of dynamic stiffness. Voltage THD allows a comparison of performance of various inverters for a same load, for example, the full non-linear load according to IEC 62040-3.
Output dynamic stiffness is typically a function of control strategy, modulation, DC-link voltage, and design of the inverter filter. In transformer-based UPS systems, the coupling of transformer windings is an additional factor.